In the past, as rubber excellent in oil resistance, heat resistance, and ozone resistance, a nitrile group-containing highly saturated copolymer rubber (also called “highly saturated nitrile rubber” and including hydrogenated nitrile rubber) has been known. In cross-linked form, it is being used as a material for belts, hoses, gaskets, packings, oil seals, and other various automobile-use rubber products. In recent years, automobile engines have been made increasingly smaller in size and higher in output. To deal with such a situation, further improvement in the heat resistance is being sought from the nitrile group-containing highly saturated copolymer rubber used for the same. Further, recently, the material has been increasingly used in cold areas such as Northern Europe and Russia. Much further improvement of the cold resistance has been sought.
As opposed to this, Patent Document 1 discloses a rubber composition improved in cold resistance comprised of a nitrile group-containing highly saturated copolymer rubber having contents of bonded monomer units in the copolymer chain of (1) unsaturated nitrile-based monomer units: 10 to 40 wt %, (2) unsaturated carboxylic acid ester-based monomer units: 1 to 40 wt %, (3) conjugated diene-based monomer units: 20 wt % or less, and (4) hydrogenated conjugated diene-based monomer units: balance, the total of the monomer units (1) and the monomer units (2) being 30 to 50 wt % and the total of the monomer units (3) and monomer units (4) being 50 to 70 wt %, and of compounding agents.
However, the cross-linked rubber obtained using the rubber composition disclosed in this Patent Document 1 is considerably improved in cold resistance, but is not sufficient in heat resistance. Much further improvement of the heat resistance and cold resistance has been desired.